The present invention relates to a semiconductor surge absorber for protecting an electrical apparatus and an electronic circuit, etc.: from overvoltage.
In an electronic apparatus using the electric and electronic circuits, a circuit or a protection element with overvoltage protection function is used to prevent damage of the electronic circuit and an electronic component by an instantaneous surge overvoltage. As overvoltage surges, there are an external lightning surge arising by lightning and abnormality of an electric power distribution system coming from an electric distribution line, and an internal lightning surge arising by on and off spark abnormality of switches, etc. inside the apparatus. A protection circuit and a protection element responding to each lightning surge are used in order to ensure a safety of the electronic apparatus.
The elements which protect the apparatus by the absorption of the overvoltage surge are named varistor or arrester collectively. For the external lightning surge, a grain boundary barrier-type varistor which consists of a ceramic made from sintered crystallites that have a main component of ZnO or SiC(silicon carbide) is widely used. The grain boundary barrier-type varistor becomes a cheap and high-performance arrester, because the varistor is using nonlinear characteristics of a ceramic resistor in which resistance value sharply decreases beyond a certain electric field intensity and it is comparatively simple to be manufactured. Therefore, this varistors is applied in great numbers not only as arresters of an electrical apparatus field but also as protection vessels of an electronic apparatus field such as a personal computer and a communicator. However, the conventional varistor has problems on its performance. One of the problems is a constant voltage characteristic which is related to an operation resistance. Since a large inside voltage drop is generated in a surge current energization by the resistance component of the ceramic, a large voltage difference occurs between an operation starting voltage (build-up voltage) and an maximum operation voltage (supreme voltage, namely maximum voltage which an element can protect). The voltage difference increases as a large-capacity element, and it reaches about xc2xd of operation starting voltage for a high power element. Therefore, it is necessary to set a voltage endurance design of an electrical apparatus at a considerably large voltage in comparison with an actual operating voltage. Consequently, not only enlargement and high price but also efficiency lowering by the increase in the power loss of the apparatus is a large problem against resource conservation and energy saving. Another problem concerns a repeated operation. Repetition constant voltage characteristic of the varistor of the sintering resistor is not secured because of the temperature characteristic of the resistor. Therefore, the application of the varistor is limited to the application which protects the apparatus from the instant surge voltage, which is one-shot like the arrester and the protection vessel. The varistor hardly can be applied to the case in which it is protected from the overvoltage which generates the application, for example the semiconductor element in the inverter apparatus, of which the overvoltage is repeatedly applied in the inside. For overvoltage countermeasure in such apparatus, a snubber circuit with resistance and snubber circuit composed of a resistor and a capacitor, etc. or a circuit having a clamp function of the voltage is connected with the semiconductor element, and the voltage endurance of the semiconductor element is set high over the double of source voltage. However, there is a problem of causing enlargement, high price and increase in power loss of the apparatus.
As a surge absorber which overcomes the problem in the varistor of the sintering resistor, the element called a junction type varistor using the breakdown phenomenon of a pn junction of Si has been used practically. For example, this element has been described, Inst. of Electronics, Inf. and Commun. Engineering edition, xe2x80x9cthe electronic information communication handbookxe2x80x9d ninth edition, page 774 semiconductor device, second department diode. The element positively utilizes fixed breakdown voltage for reverse current of wide range, in which two kinds of breakdown phenomenon in the Si pn junction occurs, namely avalanche breakdown and Zener breakdown. The element is designed so that the function and structure is suitable for the breakdown characteristic.
FIG. 2a shows a cross section and FIG. 2b shows a voltage-current characteristic of an epitaxial type voltage regulator diode using the avalanche breakdown phenomenon. A p type poly crystal Si is selectively grown on n type single crystal silicon. By a concentration gradient near pn junction that is formed with heat treatment diffusing a p type impurity in Si after the epitaxial layer formation and an impurity in the n type single crystal silicon as a substrate, the delicate control of the breakdown voltage is possible. The p type impurity layer can also be controlled by an ion-implantation and a afterwards diffusion. The breakdown region causes the avalanche breakdown in the almost fixed voltage like FIG. 2b and the reverse current remarkably increases, when the reverse-directional voltage in the diode is applied. The quality of the performance as a surge absorber is shown at an inclination (xcex94VZ/xcex94IZ)(=operation resistance Zz) of voltage (VZ) and current (IZ) of the breakdown region. The constant voltage characteristic becomes better, as this inclination becomes smaller.
A current-voltage characteristic is approximated that of the following equation.
IZ=(VZ/C)xcex1
C is a constant and xcex1 is defined herein a voltage nonlinearity index, in which xcex1=1 is usual resistance. An excellent varistor is provided as xcex1 is bigger. The varistor of the Si surge absorber has xcex1=100xcx9c500, while the above mentioned sintering resistor has xcex1xcx9c50. In addition, the Si surge absorber is widely used as a protection element of electronic apparatus, because the surge absorber endures the repetition operation of only small power and only low temperature. However, there is a problem that a surge endurance of the conventional Si surge absorber is remarkably smaller than that of the varistor of sintered resistor. The application of the Si surge absorber is comparatively limited to the surge absorber in the electronic apparatus of small capacity, since the Si surge absorber has an operating voltage of a few Vxcx9cseveral hundred V and a peak pulse current of about 100 A and an operating voltage of a few Vxcx9cseveral hundred V. The surge absorber needs to absorb a large energy, which is instantaneous. The Si surge absorber has the low operation supreme temperature of the pn junction of usual 150xcx9c200xc2x0 C., and a heat capacity of Si is comparatively small. Therefore, the absorbed energy allowed in the Si surge absorber is remarkably more limited than the ceramic varistor.
As described above, within the conventional technology, the element with excellent surge absorber function, that the flat rate of the voltage is good since the operation resistance is low, that the repetitive operation is possible, and that it stands the use of wide voltagexe2x80x94current region since the surge endurance is large, does not exist.
This invention, in which the above problems are considered, has the following objects.
An object of present invention is to offer a semiconductor surge absorber which has a large surge endurance, and in which a repeated operation is possible.
An another object of present invention is to offer an electric apparatus, an electronic apparatus, and a power module, which have a high performance, using a semiconductor surge absorber.
A semiconductor surge absorber according to an invention has a semiconductor substrate having a semiconductor single crystal with bandgap energy not less than 2.0 eV. The semiconductor substrate has a first semiconductor layer of a first conductivity type, a second semiconductor layer which forms a pn junction with the first semiconductor layer, and a third semiconductor layer which forms another pn junction with the first semiconductor layer. The second semiconductor layer is electrically connected to an electrode, and the third semiconductor layer is electrically connected to another electrode. In addition, a punchthrough voltage is lower than an avalanche voltage in each of the pn junction and the another pn junction. The first conductivity type and the second conductivity type are respectively p type or n type, and opposite conductivity types with respect to each other. Therefore, the semiconductor surge absorber according to the invention has bi-directionality, because the fundamental junction geometry of the semiconductor substrate becomes pnp or npn.
Since the semiconductor material is the single crystal in the semiconductor surge absorber according to the invention, an excellent characteristic of a large voltage nonlinearity index xcex1 in a conventional surge absorber using the single crystal silicon is retained. In addition, since the semiconductor material is wide bandgap semiconductor with bandgap energy not less than 2.0 eV, the operation supreme temperature and the crystal melting temperature are high, and the thermal conductivity is high. Therefore, the semiconductor surge absorber has the large surge endurance and the repeated operation.
In addition, a surge absorption operation starting voltage in which a current suddenly flows out is decided by the punchthrough voltage, because the punchthrough voltage is lower than the avalanche voltage in each pn junction of the semiconductor surge absorber. The punchthrough voltage is set by impurity total amount of a semiconductor layer which forms a pn junction. Therefore, even if setting an accurate avalanche voltage of a wide bandgap semiconductor is difficult for a property of difficulty of impurity diffusion in the semiconductor, the operation starting voltage is set with high-precision. It is desirable that impurity concentration of the second and third semiconductor layers is larger than the first semiconductor layer and that the punchthrough voltage is set by a punchthrough of a depletion layer in the second and third semiconductor layer. By this, the surge absorption operation starting voltage is high-precisely set by impurity total amount introduced into the semiconductor substrate, when the second and third semiconductor layers are formed.
In the case of applying an avalanche breakdown instead of a punchthrough breakdown, the wide bandgap single crystal semiconductor also carries a large surge endurance and a repeated operation for a semiconductor surge absorber. Additionally, an operation starting voltage of a semiconductor surge absorber having a semiconductor substrate having a semiconductor single crystal with bandgap energy less than 2.0 eV such as Si is also set with high-precision with applying the punchthrough breakdown.
A semiconductor surge absorber made from single crystal semiconductor material with the bandgap energy not less than 2.0 eV is connected to the electrical and electronic apparatus according to the invention. A large margin for a source voltage or a continuous operating duty voltage become unnecessary in setting of a blocking voltage of the electrical and electronic apparatus, since the connected surge absorber has a high surge endurance and it operates repeatedly by the wide bandgap single crystal semiconductor material. Therefore, the electrical and electronic apparatus are miniaturized, and the power loss is reduced. Especially, an effect of the invention is remarkable, when the high voltage is handled. New electrical and electronic apparatus with a source voltage or a continuous operating duty voltage not less than 220 V and a blocking voltage from 100-150% of a peak value of the source voltage or the continuous operating duty voltage are realized. An electrical apparatus such as a power converter and a various power source and an electronic apparatus such as a communication device and a terminal apparatus and a computer are included on the electrical and electronic apparatus according to the invention.
A power module according to the invention has a metal plate, a circuit board bonded on the metal plate, and a semiconductor switching element and a semiconductor surge absorber connected to each other in parallel by mounting in the metal plate and the circuit board. When a voltage applied to the semiconductor switching element in the power module rises, the applied voltage is clamped at punchthrough voltage of an operation starting voltage in the semiconductor surge absorber. Therefore, a large margin for a source voltage or a continuous operating duty voltage become unnecessary in setting of a blocking voltage of the semiconductor switching device. Concretely, a blocking voltage of the semiconductor switching element is set to 100-150% of the peak value of source voltage. Therefore, the power loss of the power module is reduced. In electrical and electronic apparatus according to the invention, a similar action and effect occur by parallel connecting a semiconductor surge absorber including the semiconductor surge absorber according to the invention, which is made from a wide bandgap semiconductor single crystal material, when the apparatus have the semiconductor switching element.